1. Field of the Invention
The present invention relates to transmission modules that include an amplifier circuit, a non-reciprocal circuit, and an interstage matching circuit connected between an output end of the amplifier circuit and an input end of the non-reciprocal circuit.
2. Description of the Related Art
In recent years, there has been a demand for multiband and multimode portable communication terminals, such as cellular phones and portable information terminals, which are compatible with a plurality of communication modes using mutually different frequency bands or modulation methods. Thus, in a transmission module mounted in a transmission section of such a portable communication terminal, as illustrated in FIG. 8, a non-reciprocal circuit 300 having broadband isolation characteristics for a plurality of high frequency signals of mutually different frequency bands is provided (e.g., Patent Document 1). The non-reciprocal circuit 300 illustrated in FIG. 8 includes an isolator 130, a broadband circuit 4 that is connected in parallel to the isolator 130, and an output matching circuit 5 that is connected in series to the isolator 130 at a subsequent stage thereof. The isolator 130 includes a microwave ferrite 31 having a pair of principal surfaces located so as to be opposite to each other, and a first center electrode (inductor L1) and a second center electrode (inductor L2) are provided on the ferrite 31. The first center electrode is connected at one end to an input port of the ferrite 30 and connected at the other end to an output port of the ferrite 31, and the second center electrode is connected at one end to the output port and grounded at the other end in a state in which the second center electrode is isolated from the first center electrode on both principal surfaces. Then, a direct current magnetic field is applied to a portion where the first center electrode (inductor L1) and the second center electrode (inductor L2) intersect with each other through permanent magnets.
The broadband circuit 4 includes a capacitor C1 that is connected in parallel to the inductor L1 (first center electrode), and a series circuit that is connected in parallel to the inductor L1 and formed by a terminator R and an LC series resonant circuit (inductor L3 and capacitor C3). The output matching circuit 5 includes a capacitor CS2 that is connected in series to the output port of the isolator 130, and a capacitor C2 that is connected to the output port of the isolator 130 so as to be in parallel with the inductor L2. In addition, an input matching circuit 6 that includes a matching capacitor CS1 is connected to an input end of the non-reciprocal circuit 300.
In the non-reciprocal circuit 300 illustrated in FIG. 8, when a high frequency current is inputted to the input end through the input matching circuit 6, a large high frequency current flows through the second center electrode (inductor L2), whereas the high frequency current rarely flows through the first center electrode (inductor L1). Thus, an insertion loss is small, and the non-reciprocal circuit 300 operates in a broad band. At this point, the high frequency current rarely flows through the terminator R and the LC series resonant circuit (inductor L3 and capacitor C3), either. In the meantime, when a high frequency current is inputted to an output end of the non-reciprocal circuit 300, the high frequency current is matched in a broad band through the impedance characteristics of the terminator R and the LC series resonant circuit formed by the inductor L3 and the capacitor C3, and thus the isolation characteristics of the non-reciprocal circuit 300 improve. It is to be noted that the center frequency of the non-reciprocal circuit 300 is determined by a parallel resonant circuit that is formed by the capacitors C1 and C3 and the inductor L3 of the broadband circuit 4 and the inductor L1 (first center electrode).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-302742 (paragraphs 0014 to 0018, FIG. 1, Abstract, etc.)